dashboard: skip dev.boringcrypto branches on most builders

We care most about testing the dev.boringcrypto branches on linux/amd64.
Stop testing them on all builders, as it's not worth the resource use.

Continue to test on one additional non-linux/amd64 port,
namely windows/386, to ensure that fallbacks work.

Fixes golang/go#26791

Change-Id: I4556886f55175173de52e35cf57833b22843d586
Reviewed-on: https://go-review.googlesource.com/c/149739
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

dashboard/builders.go

@@ -869,6 +869,12 @@ func (c *BuildConfig) BuildBranch(repo, branch, goBranch string) bool {
 			return false
 		}
 	}
+	// Build dev.boringcrypto branches only on linux/amd64 and windows/386 (see golang.org/issue/26791).
+	if repo == "go" && (branch == "dev.boringcrypto" || strings.HasPrefix(branch, "dev.boringcrypto.")) {
+		if c.Name != "linux-amd64" && c.Name != "windows-386-2008" {
+			return false
+		}
+	}
 	return true
 }